Heretofore, when cutting off a workpiece, such as a steel plate, by making use of a laser processing machine, O.sub.2 gas having a pressure of 0.5 to 6 kg/cm.sup.2 has been used as an assist gas under such a condition as focusing a laser beam onto an upper surface of the workpiece. On the other hand, when cutting off a workpiece, such as a thin plate having the thickness of less than 1 mm, air has been used also as an assist gas, because a little dross is attached to the workpiece.
O.sub.2 gas can be used as an assist gas without causing any problem when a workpiece is made of soft steel. However, when cutting a workpiece made of material having a high viscosity in its melted state such as, for example, stainless steel, aluminum, and so forth, if the gas is used, it does not become possible to obtain a fine cut surface, because dross generated upon cutting the workpiece is attached to the cut surface.
In order to improve the above mentioned inexpediency, a pressure of assist gas may be raised so as to blow off the dross. However, if the pressure of assist gas is raised, there has been a disadvantage in that a volume of assist gas increases with the result that a costs increase.
On the other hand, when using air as an assist gas, dross tends to attach to a cut surface of a workpiece, because a degree of oxidization of the dross is low as compared with the case in which O.sub.2 gas is used as an assist gas. As a result, it does not become possible to obtain a fine cut surface of the workpiece to be cut. Therefore, there has been such an inexpediency as being necessary to require many working steps as an aftertreatment such as, for example, an application of machining on the cut surface of the workpiece.